1. Field of the Invention
This invention relates to systems and methods for continuously measuring and monitoring the characteristics of a moving filament, such as the denier of a synthetic yarn, by passing the filament through a capacitive sensor to develop an electrical signal representing an absolute measurement of the filament, with reference to a prescribed datum or zero point. For example, denier is a unit of fineness for yarn equivalent to 1 gram per 9,000 meters of length, referred to zero. Thus a 15-denier weighs 15 grams per 9,000 meters. More particularly, this invention relates to a means and method for abrogating errors arising from slowly occurring variations in the capacitive sensor.
2. Description of the Prior Art
Devices and methods for capacitively monitoring the characteristics of a continuously moving filament are known. In one advantageous device, disclosed in U.S. Pat. No. 3,879,660 to Piso, a filament is passed through a capacitive sensor to develop an absolute measurement of the filament with reference to a prescribed datum or zero point, thus permitting the monitoring of characteristics such as the denier of synthetic yarn filaments, with the absolute measurement of denier being made available for utilization, e.g., to give an alarm if the denier measurement is outside a prescribed range of acceptable deniers.
An example of the use of such a filament monitoring device is shown schematically in FIG. 1, wherein a filament F is extruded from an extruding head E and is to be wound upon a bobbin B. The filament F passes through a slot S in a capacitive sensor head H, which is arranged to supply, on output line L, an electrical signal which varies with the capacitance of filament F, and thereby provides a measurement of the filament's denier. It has been learned that as filaments are monitored in sensor heads H, contaminants from various sources build up between the capacitor plates in sensing head H, causing the signal on line L to drift and no longer provide an accurate absolute measurement of the capacitance of filament F.
Heretofore errors due to the buildup of contaminants in sensor heads H has been counteracted by periodic cleaning of the sensor heads. However, relatively common-place filaments F promote a rapid contaminant buildup and necessitate frequent cleaning. For example, low denier filaments may contain agents which contaminate the sensor head and require it to be examined and cleaned as much as twice a week. High denier filaments, such as those used for tire cords, are subject to flaking, and sometimes have an oil finish, which lead to rapid contaminant buildup and require more frequent cleaning.
Cleaning of sensor heads necessarily requires substantial interruption of monitoring, and prevents full utilization of capacitive measurement systems in filament monitoring if accurate absolute measurements are to be maintained.